Oam pseudo-doppler receiving architecture

ABSTRACT

The disclosed systems, structures, and methods are directed to an orbital angular momentum (OAM) receiver. The OAM receiver includes at least two receiver antenna elements to receive radiated OAM signal beams containing superposed order modes and to generate antenna element output signals based on the received OAM signal beams. The receiver antenna elements are positioned tangentially along a circular locus and spatially separated by a distance. A variable ratio combining unit operates to switch between the antenna output signals based on a high-rate periodic waveform that emulates unidirectional movement by the antenna elements to produce a pseudo-Doppler frequency shift. The variable ratio combining unit further modulates the antenna output signals based on the periodic waveform to impart a fractional pseudo-Doppler shift to each OAM mode and combines the modulated antenna element output signals in accordance with the fractional pseudo-Doppler shift to facilitate separation of the OAM modes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 62,748,226, filed on Oct. 19, 2018 and entitled “OAM Pseudo-Doppler Receiving Architecture”, the contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to the field of radio-frequency (RF) communications, in particular, to systems and methods directed to applying pseudo-Doppler techniques to substantially enhance the processing fidelity and accuracy of received orbital angular momentum (OAM)-based RF communication links.

BACKGROUND

In view of the proliferation of wireless communication usage, numerous proposals have been presented regarding the improvement of service facilities for existing wireless communication systems as well as for next-generation wireless communication systems. Many of the proposed improvements call for the enhanced capabilities and increased implementation of multiple-input, multiple-output (MIMO) and massive-MIMO (M-MIMO) receiver architectures.

To this end, orbital angular momentum (OAM)-based radio-frequency (RF) signals offer an additional spatial dimension, namely, an additional degree of freedom, which can be exploited to enhance the capacity of wireless communication links.

However, conventional implementations of OAM-based RF communications have demonstrated certain deficiencies regarding the effective recovery of OAM signals at far-field distances.

SUMMARY

An object of the present disclosure is to provide an orbital angular momentum (OAM) receiver architecture and system. The disclosed system includes at least two receiver antenna elements configured to receive radiated OAM signal beams containing superposed k order modes and to generate antenna element output signals based on the received OAM signal beams, in which the receiver antenna elements are positioned tangentially along a circular locus and spatially separated by a distance d. A variable ratio combining unit combines the antenna element output signals in time-varying proportions. The variable ratio combining unit is configured to switch between portions of the antenna element output signals based on a high-rate periodic waveform of frequency F, the high-rate switching operation emulating unidirectional movement by the antenna elements to produce a pseudo-Doppler frequency shift. The variable ratio combining unit further modulates and time-gates the antenna element output signals based on the high frequency periodic waveform to impart a fractional pseudo-Doppler shift to each OAM mode and combines the modulated and time-gated antenna element output signals in accordance with the fractional pseudo-Doppler shift to facilitate separation of the OAM modes encompassing the streams of information data symbols.

A further object of the present disclosure is to provide a method for processing received orbital angular momentum (OAM) signals. The disclosed method includes receiving, by at least two receiver antenna elements, radiated OAM signal beams containing superposed k order modes wherein each of the K modes encompasses an individual stream of information data symbols, in which the receiver antenna elements are positioned tangentially along a circular locus and spatially separated by a distance d and a circular locus with a radius R corresponding to a footprint area of the received OAM signal beams, such that the circular locus contains progressive phase gradient information along a circumference of the circular locus. Antenna element output signals are generated based on the received OAM signal beams and are combined by a variable ratio combining unit in time-varying proportions. Portions of the antenna element output signals are switched in accordance with a high-rate periodic waveform of frequency F, the high-rate switching operation providing emulation of unidirectional movement by the receiver antenna elements along the circumference of the circular locus to produce a pseudo-Doppler frequency shift. The antenna element output signals are modulated and time-gated in accordance with the high frequency periodic waveform to impart a different pseudo-Doppler shift to each OAM mode. The modulated and time-gated antenna element output signals are then combined in accordance with the fractional pseudo-Doppler shift to facilitate separation of the OAM modes encompassing the streams of information data symbols.

BRIEF DESCRIPTION OF THE FIGURES

The features and advantages of the present disclosure will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

FIG. 1A (Prior Art) depicts a high-level functional block diagram of a conventional OAM RF generating architecture;

FIG. 1B (Prior Art) depicts a three dimensional graph of representative far-field OAM RF beam patterns;

FIG. 2A illustrates a conceptual view of a pseudo-Doppler scheme, in accordance with various embodiments of the present disclosure;

FIG. 2B depicts an interpolated phase between antenna element signals representative of the pseudo-Doppler effect;

FIG. 2C depicts a representative control waveform configured to switch between receiving element signals;

FIG. 3A depicts a high-level functional block diagram of a variable ratio power combining unit, in accordance with various embodiments of the present disclosure;

FIG. 3B depicts another variable ratio power combining unit that incorporates a gating unit at the output to emulate unidirectional antenna element motion, in accordance with various embodiments of the present disclosure;

FIG. 4A illustrates a received signal constellation plot for an OAM8 beam at a 0 Hz down-conversion frequency shift, in accordance with various embodiments of the present disclosure;

FIG. 4B illustrates spectral characteristics for the OAM8 beam at the 0 Hz down-conversion frequency shift, in accordance with various embodiments of the present disclosure;

FIG. 4C illustrates a received signal constellation plot for an OAM1 beam at the 0 Hz down-conversion frequency shift, in accordance with various embodiments of the present disclosure;

FIG. 4D illustrates spectral characteristics for the OAM1 beam at the 0 Hz down-conversion frequency shift, in accordance with various embodiments of the present disclosure;

FIG. 5A illustrates a received signal constellation plot for an OAM8 beam at a 2F down-conversion frequency shift, in accordance with various embodiments of the present disclosure;

FIG. 5B illustrates spectral characteristics for the OAM8 beam at the 2F down-conversion frequency shift, in accordance with various embodiments of the present disclosure;

FIG. 5C illustrates a received signal constellation plot for an OAM1 beam at the 2F down-conversion frequency shift, in accordance with various embodiments of the present disclosure;

FIG. 5D illustrates spectral characteristics for the OAM1 beam at the 2F down-conversion frequency shift, in accordance with various embodiments of the present disclosure;

FIG. 6A illustrates a representative spectral envelope and replicas for OAM8, in accordance with various embodiments of the present disclosure;

FIG. 6B illustrates a representative time-limited frequency-shift structure for the recovery of OAM modes, in accordance with various embodiments of the present disclosure;

FIG. 7 illustrates an extended variable ratio power combining unit that incorporates a gating unit at the output to emulate unidirectional antenna element motion, in accordance with various embodiments of the present disclosure;

FIG. 8A illustrates a high-level functional flow diagram of an OAM signal recovery process, in accordance with various embodiments of the present disclosure; and

FIG. 8B illustrates a detailed functional flow diagram of the OAM signal recovery process, in accordance with various embodiments of the present disclosure.

It is to be understood that throughout the appended drawings and corresponding descriptions, like features are identified by like reference characters. Furthermore, it is also to be understood that the drawings and ensuing descriptions are intended for illustrative purposes only and that such disclosures are not intended to limit the scope of the claims.

DETAILED DESCRIPTION

As used herein, the term “about” or “approximately” refers to a +/−10% variation from the nominal value. It is to be understood that such a variation is always included in a given value provided herein, whether or not it is specifically referred to.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the described embodiments appertain.

It should be understood that OAM RF waves are configured to manifest various orders of OAM modes, denoted by integers ±k. The OAM RF waves are generated by imposing a phase shift of k2π radians for every revolution of the observation point around the beam axis to produce a helical “corkscrew-shaped” waveform front. This may be achieved by using a uniform circular array of K identical antenna elements, wherein each of the K elements is fed by a current that is shifted in phase from that of its neighboring element in one direction by k2π/K radians at the same amplitude.

As such, FIG. 1A (Prior Art) illustrates a high-level functional block diagram of a conventional OAM RF receiving architecture 100. As depicted, OAM architecture 100 comprises a circular array 110 of K antenna elements a₁-a_(K) and a Butler Matrix structure 120 having K input ports C₁-C_(K) and K output ports P₁-P_(K). (For the sake of simplicity, K=4 in FIG. 1A). The architecture 100 operates in the receive mode to sense multiple OAM beam excitations along the circular array of K antenna elements. This is achieved by coupling the K antenna elements a₁-a_(K) to the K input ports C₁-C_(K) of the Butler Matrix structure and coupling the K output ports P₁-P_(K) to the K receivers in the same RF band. Each of the K OAM beams is modulated by a different stream of independent data symbols, which are fed to a separate receiver. Being reciprocal, the same structure works in the transmitting mode, with the receivers replaced by transmitters, each of which is modulated by a different stream of independent data symbols.

FIG. 1B (Prior Art) depicts a three-dimensional graph of representative far-field OAM RF beam patterns 150. As shown, the OAM beam patterns exhibit a conical shape for all non-zero k orders having “vortex-shaped” axial nulls. The shade gradations indicate the electrical phase at a fixed time, modulo-2π radians, in which the phase patterns rotate around the beam axis at the RF rate in time (i.e. one revolution per cycle at RF). In so doing the k phase fronts, as shown by the repeating shade gradations, pass a point on the cone of the k-th OAM beam along the tangential direction, per period of the RF carrier wave. Equivalently, at any given point in time, an electrical phase gradient of k2π/(2πR) radians per meter exists along the circular locus of radius R around the axis of the conical beam of the k-th order OAM mode.

As noted above, the non-zero k order OAM beam patterns 150 manifest “vortex-shaped” axial nulls at far-field distances, which is typically where conventional receiving antennas/apparatus are positioned. Furthermore, the conventional receiver processing of OAM beams generally rely on spatial techniques employing the reciprocal principles used to generate the OAM modes at the transmitter.

As a result, most attempts at exploiting the OAM modes to enhance the capacity of radio links suffer from low signal-to-noise ratios (SNRs) and acute sensitivity to crosstalk issues due to position errors. Moreover, such attempts impose implementation restrictions on receiving antennas/apparatus, such as requiring the use of large receiving antennas, operational constraints associated with any or all of very short wavelengths and limited range distances.

OAM Psuedo-DOPPLER Receiver Scheme and System Architecture

The present disclosure provides an OAM RF receiver scheme and architecture that implements a pseudo-Doppler technique. The OAM pseudo-Doppler architecture refers receiver systems, devices, and other structures embodying the pseudo-Doppler technique. This technique provides a frequency domain-based solution to obviate or mitigate the above-noted limitations of conventional receiver schemes. As will be described in greater detail below, the disclosed embodiments provide for a pseudo-Doppler scheme that operates to enable toggling or gradual switching between signals outputted by at least two fixed, spatially-separated receiving antenna elements to artificially emulate a unidirectional antenna movement commensurate with traditional Doppler-based processing.

The emulated antenna movement is achieved by a rapid, periodic, modulating waveform controlling a variable ratio RF power combining (VRPC) unit that drives the toggling between the signals outputted by the antenna elements within defined time intervals. The VRPC unit subsequently combines the modulated antenna element signals and time-gates those signals for further processing designed to separate and demodulate the received OAM beams into meaningful payload data.

OAM Psuedo-DOPPLER Scheme

In traditional Doppler-based RF direction-finding applications, physically moving antennas are used to resolve angular direction based on detected frequency shifts of the received signal. By way of brief summary, such Doppler-based applications employ antenna element(s) that physically move (i.e., rotate) along a circular locus at a constant tangential velocity. The tangential velocity imparts a proportional Doppler frequency shift, which is imposed on the signal received at the antenna element(s) to frequency modulate (FM) the received signal. The FM signal manifests a deviation equivalent to the frequency shift and a phase corresponding to the azimuthal direction of the arriving received signal. The azimuthal angular direction is then resolved as a function of the FM phase information.

With this said, the disclosed embodiments present a scheme that exploits the principles noted above to artificially emulate the physical rotating motion of the antenna element(s) and create a pseudo-Doppler effect based on the phase gradient of an on-axis OAM beam. The pseudo-Doppler effect imparts a Doppler frequency shift that is proportional to the order k of each of the OAM modes of the received signals, thereby facilitating mode separation processing and subsequent extraction of payload data.

In particular, FIG. 2A illustrates a conceptual view of pseudo-Doppler scheme 200, in accordance with various embodiments of the present disclosure. The depicted annular circular ring represents the footprint area of a k-th order OAM received conical beam, in which the shade gradations indicate spatial phase progression, or gradient, at a given time instant along the circular locus of the footprint annular ring defined by radius R. As shown, scheme 200 employs two receiver antenna elements RX1, RX2 that are fixedly positioned tangentially to the circular locus and are spatially separated by a distance d. With this arrangement, the physical movement of traveling along the circular locus may be artificially emulated by gradually switching between antenna elements RX1, RX2, in one direction, repeatedly.

That is, as shown in FIG. 2A, antenna elements RX1, RX2 are fixedly positioned along the circular locus separated by distance d. However, by rapidly and periodically toggling between portions of the signal outputted by elements RX1, RX2 over time, the appearance of unidirectional circumferential movement is achieved. The emulated circumferential movement is indicated by the interpolated element positions and the shaded arrow depicted in FIG. 2A. So, as the emulated movement appears to travel from one interpolated position to another, the corresponding changes in phase incurred by the emulated movement along the phase gradient produces a pseudo-Doppler frequency shift effect.

FIG. 2B depicts a representative virtual antenna phase ψ(t) effected by a control waveform P(t) configured to drive the rapid, periodic switching between the signals outputted by antenna elements RX1, RX2. As shown, control waveform P(t) manifests a “saw-tooth” or unidirectional profile that rapidly and periodically toggles between the elements RX1, RX2 signals having respective phases ϕ₁ and ϕ₂.

It will be appreciated that the principles and concepts presented by the instant disclosure are not limited to the use of the control waveform P(t) noted above and implied by FIG. 2B, as other suitable waveforms capable of rapid, unidirectional, and periodic switching may be used. For example, FIG. 2C provides an alternative control waveform P_(s)(t). P_(s)(t) comprises orthogonal sinusoidal waveforms in which a sine modulation is applied to one antenna element output signal and a cosine modulation is applied to the other antenna element output signal, with suitable time-gating to emulate directionality in the final output signal, as in the embodiment depicted in FIG. 3B.

With regard to the time-gating functionality, it will be noted that the necessity of such functionality is a consequence of the limits of the region of validity relative to the pseudo-Doppler frequency shift when using the sine and cosine control waveforms where P(t)=Ωt. The regional limits of validity are periodic, the fundamental period in terms of phase being:

$\begin{matrix} {\frac{- \pi}{2} < {\Omega \; t} < 0} & (1) \end{matrix}$

which repeats at intervals of ±nπ as shown in FIG. 2C. Thus, the time-gating should occur periodically in real time to ensure the desired frequency shifts in the final output and should be synchronous with the pseudo-Doppler modulation.

It follows that the gating intervals are designed to contain those portions of the modulations which cause one antenna output to be increasing and the other decreasing the magnitude of its contribution to the final output signal. As shown in FIG. 2C, the alternating signs of the gating waveform ensure that always the same antenna output is increasing while the other is decreasing. In so doing, pseudo-Doppler scheme 200, performs the desired interpolation that emulates a unidirectionally moving antenna between the two stationary receive antennas.

OAM Pseudo-DOPPLER System Architecture

As depicted in FIG. 2A and noted in the description of pseudo-Doppler scheme 200, the signals outputted by antenna elements RX1, RX2 are supplied to a variable ratio power combining (VRPC) unit 300. VRPC unit 300 is configured to drive the switching between the signals outputted by elements RX1, RX2 to emulate unidirectional antenna movement based on a rapid, periodic, control waveform as well as combine the outputted signals. This emulated movement must be sufficiently rapid to effect a pseudo-Doppler shift that is at least as large as the bandwidth of each OAM mode modulation signal, so the OAM mode signals can be separated in frequency. This movement typically far exceeds any physically-realizable actual movement of the RX antenna elements.

In view of scheme 200 described above, it will be appreciated that the reference phase (relative to a fixed reference position on the footprint) for the emulated moving antenna element between RX1, RX2 may be expressed as:

ϕ_(n) =kθ _(n)  (2)

where k is the OAM mode order and θ_(n) is the azimuthal angular position of the receiver element. It follows that, as antenna elements RX1, RX2 emulate movement of one antenna around the circular locus at a uniform velocity v, its emulated angular position changes linearly with time, thereby causing corresponding phase value changes in its output that also vary linearly with time. The time-based phase variances may be expressed as:

$\begin{matrix} {\frac{d\; \phi_{n}}{dt} = {\frac{{kd}\; \theta_{n}}{dt} = {\frac{{kv}_{n}}{R} = {2\pi \; f_{n,{Doppler}}}}}} & (3) \end{matrix}$

in which

$\begin{matrix} {v_{n} = {{\frac{d}{dt}\left( {R\; \theta_{n}} \right)\mspace{14mu} {and}\mspace{14mu} 2\pi \; f_{n,{Doppler}}} = \frac{k\; d\; \theta_{n}}{dt}}} & (4) \end{matrix}$

where R is the radius of the circular footprint locus. Thus, the spatial-domain properties of the OAM k-order beams include phase gradient information k/R which, by virtue of the emulated motion of antenna elements RX1, RX2, may be transformed to frequency-domain characteristics, namely, transverse Doppler shift f_(n,Doppler). It will be noted that Doppler shift f_(n,Doppler) is directly proportional to the received OAM mode k while remaining independent of the RF carrier frequency. It will be appreciated that the disclosed embodiments aim to replicate such a Doppler shift by replacing the role of the antenna velocity v with a toggling action between two separate but fixed antennas at a rate proportional to F, which is designated as the pseudo-Doppler frequency.

Given this context, the phases of the RF waves embodied by the OAM mode k beams received by antenna elements RX1, RX2 referenced as ψ₁, ψ₂ advance at k multiples of 2π radians for one complete cyclical trip around the circular footprint 2πR, at any given point in time. As such, the phases ψ₁, ψ₂ differ by kd/(2πR) for a portion of the footprint covered by the antenna element separation d. Therefore, the relationship between the respective phases ψ₁, ψ₂ of receiver antenna elements RX1, RX2 at time t may be expressed as:

$\begin{matrix} {{\psi_{2}(t)} = {{\psi_{1}(t)} - {k\; 2{\pi \left( \frac{d}{2\pi \; R} \right)}}}} & (5) \end{matrix}$

with the relationship between phases ψ₁, ψ₂, the inputs to VRPC unit 300, referenced as W₁, W₂, may be modeled, as follows:

W _(1,k)(t)=S _(k)(t)e ^(jψ) ¹ ^((t))

W _(2,k)(t)=S _(k)(t)e ^(jψ) ² ^((t)) S _(k)(t)e ^(jψ) ¹ ^((t)−jkd/R)  (6)

where S_(k)(t) is the signal of the k-th OAM beam received at the reference point in the far field. Furthermore, when P(t)=Ωt, Ω=2πF is the radian pseudo-Doppler frequency, F is the corresponding frequency in Hz, and the far-field condition kd/(2R)<<π/4 is met, one of the two outputs of VRPC unit 300, referenced as Z_(1,k)(t), may be approximated at selected gating time intervals by:

$\begin{matrix} {{{Z_{1,k}(t)} \approx {\sqrt{2}{S_{k}(t)}e^{j{({{\psi_{1}{(t)}} - \frac{kd}{2R}})}} \times \Lambda}}{\Lambda \times \left( {\cos \left( {{\Omega \; t} + {\pi/4}} \right)} \right)e^{{j{(\frac{kd}{2R})}}{({{\Omega \; t} + {\pi/4}})}}}} & (7) \end{matrix}$

Armed with these relationships, the implementation of VRPC unit 300 may be realized. To this end, FIG. 3A illustrates a high-level functional block diagram of VRPC unit 300, in accordance with various embodiments of the present disclosure. In the illustrated embodiment, VRPC unit 300 incorporates two hybrid combiners 310, 316 and two oppositely-adjusted variable phase shifters 312, 314. The variable phase shifters 312, 314 are modulated by the control waveform P(t), as noted above, at a very high rate proportional to F. The output may then be expressed as:

$\begin{matrix} {{Z_{1,k}(t)} = {{S_{k}(t)}{e^{j\; {\psi_{1}{(t)}}}\left\lbrack {{\cos \left( {P(t)} \right)} - {e^{\frac{- {jkd}}{R}}{\sin \left( {P(t)} \right)}}} \right\rbrack}}} & (8) \end{matrix}$

where the k-th OAM mode signal may be modeled as the product of a data-modulation envelope and an RF carrier phasor:

S _(k)(t)=m _(k)(t)e ^(jαπ)  (9)

As shown in FIG. 3A, the outputs of receiving antenna elements RX1, RX2 are coupled to input ports W₁ and W₂ of VRPC unit 300, respectively, and the output is taken at port Z₁. It will be understood that at least one of port Z₂ and a switching arrangement between ports port Z₁, Z₂ to a common output port could also be used, consistent with the principles of the instant disclosure pertaining to subsequent time-gating arrangements to effect unidirectionality in the pseudo-Doppler frequency shifts.

The periodic waveform P(t) is applied to a control port and operates the variable phase-shifters in opposing directions at a high rate proportional to F. In so doing, the Z₁/Z₂ output signal will appear to be shifted by a fraction of the pseudo-Doppler frequency, Fkd/(2R), which functionally corresponds to the originally desired transverse-Doppler shift, f_(n,Doppler) as represented by equation (4).

In this manner, VRPC unit 300 is capable of combining the outputs of antenna elements RX1, RX2 in time-varying proportions ranging from only RX1 output, to half of each of the RX1, RX2 outputs, to only RX2 output. This is, in effect a form of gradual switching between the antenna elements in one direction that should be repeated periodically at some rate.

FIG. 3B illustrates a high-level functional block diagram of an alternative VRPC unit 350, in accordance with various embodiments of the present disclosure. In the illustrated embodiment, VRPC unit 350 comprises two multipliers 352, 354, a hybrid combiner 356, and a switching and time-gating unit 358, 360. In this embodiment, the outputs of antenna elements RX1, RX2 are supplied to multipliers 352, 354. VRPC unit 350 is mathematically equivalent to VRPC unit 300, but effectively uses P(t)=Ωt ,where Ω=2πF to apply quadrature sinusoidal waveforms at the pseudo-Doppler frequency F to the inputs of each of multipliers 352, 354.

The multiplier 352, 354 outputs are subsequently combined by hybrid combiner 356 to yield output signals Z₁, Z₂. The switching and time-gating unit 358, 360 operates to provide a synchronous time-gating function to limit the two output signals Z₁, Z₂ to the time-intervals of validity when the desired fractional pseudo-Doppler frequency shift occurs Thus, much like the embodiment of VRPC unit 300, VRPC unit 350 yields output signal Z₁/Z₂ that appears to be shifted by a fraction of the pseudo-Doppler frequency, Fkd/(2R), which functionally corresponds to the originally desired transverse-Doppler shift, f_(n,Doppler).

OAM Pseudo-DOPPLER Simulation Results

Simulation trials were conducted based on the OAM pseudo-Doppler system architectures disclosed above. After modulation by the pseudo-Doppler waveforms, time-gating and down-conversion operations, the constellation and spectrum of the composite received signal are illustrated. The constellations pertain to the spectral replica centered at baseband (0 Hz frequency) in the spectral plots after the frequency shifting by the down-converter, reflecting a low-pass filtering operation performed in the simulation prior to demodulation. For simplicity of illustration, only one frequency-shift operation is performed in the down-converter, as opposed to the full structure of the OAM recovery scheme illustrated later in FIG. 6B.

FIGS. 4A, 4B depict the constellation plot and spectral characteristics of OAM mode 8 (OAM8) at a 0 Hz frequency shift in the down-converter, in accordance with various aspects of the present disclosure. Similarly, FIGS. 4C, 4D depict the 64 QAM constellation plot and spectral characteristics of OAM mode 1 (OAM1) at the 0 Hz frequency shift in the down-converter, in accordance with various aspects of the present disclosure.

The resulting spectral replicas of each of two simulated modes OAM8 and OAM1 that are transmitted and received separately are depicted by FIGS. 4B, 4D, respectively. The complex 64 QAM constellations are depicted by FIGS. 4A, 4C, respectively. It is apparent that each OAM mode has a different spectral envelope and that the baseband (centered at 0 Hz) spectral replica contains a different proportion of each OAM mode, as evidenced by the relative sizes of their constellations and corresponding baseband spectra.

Therefore, at a frequency shift of 0 Hz in the down-converter, OAM8 is superior to OAM1. With suitable scaling by a complex coefficient (i.e., amplitude and phase change), OAM8 could be recovered even in the presence of OAM1, and after suitable conventional equalization and decoding, its QAM data symbols may be successfully demodulated.

With a different frequency shift applied at the down-converter, different proportions of each OAM mode may be achieved and allow for the recovery of other OAM modes. Thus, FIGS. 5A, 5B depict the constellation plot and spectral characteristics of OAM8 at a 2F (i.e., twice the pseudo-Doppler modulation frequency) frequency shift in the down-converter, in accordance with various aspects of the present disclosure. Similarly, FIGS. 5C, 5D depict the constellation plot and spectral characteristics of OAM1 at the 2F frequency shift in the down-converter, in accordance with various aspects of the present disclosure.

As demonstrated by FIGS. 5A-5D, the OAM proportions of OAM8 and OAM1 are roughly the inverse of those shown in FIGS. 4A-4D. That is, at a frequency shift of 2F in the down-converter, OAM1 is superior to OAM8, so OAM1 could be similarly recovered and demodulated in the presence of OAM8.

Without any other signal processing, each OAM mode was recovered from the composite signal with a bit error rate (BER) on the order of ≈10⁻¹. In general, the differences in relative proportions of OAM modes in the various spectral replica will not be as conveniently large as illustrated in simulated trials and they will need to be recovered from several spectral replicas by jointly inverting their proportions using a matrix-vector multiplication scheme, as illustrated later in FIG. 6B. Moreover, the expected spectral shifts by fractions of the pseudo-Doppler modulation frequency appear to be absent in all of the output spectral replicas, but they are actually present in their envelopes, as will be indicated by FIG. 6A.

OAM Pseudo-DOPPLER Recovery Scheme

Based on the simulation trials, the OAM signals are present in different proportions in the various harmonic spectral replicas of the composite received signal at the output of the gating subsystem of the OAM system architecture noted above. These proportions are determined by the physical parameters of the link, which can be made known to the receiver à-priori, to enable effective recovery of the OAM modes.

Moreover, the gating pseudo-Doppler modulations of the composite received signal evidence a “time-limited fractional frequency shift” operation in the discrete frequency domain. This may be recognized as the dual of a “frequency-limited fractional time shift”, or band-limited fractional delay operation on a signal in discrete time domain. That is, the gating frequency, which is twice the pseudo-Doppler modulation frequency 2F and the fraction comprising the OAM spectral shift, Fkd/(2R), correspond to the sampling interval and the fraction thereof, respectively, in the band-limited fractional-delay operation.

The correspondence to the sampling interval and the fraction thereof may be expected based on the duality relations that exist between time and frequency domains due to properties of the Fourier transform and its inverse. The property that sampling in the time-domain at intervals of T causes periodic extensions in frequency-domain by 1/T, also explains the received spectra observed in the simulation trials. The sampling operation is analogous to the time-gating functionality according to equation (1) and as depicted in FIG. 2C.

Moreover, as discussed above regarding the simulation results of OAM8 and OAM1 and depicted by FIGS. 4A-4D and 5A-5D, each OAM mode has a different spectral envelope and the corresponding spectral replicas contain different proportions of each OAM mode, as evidenced by the relative sizes of their constellations. The proportions of OAM modes in the spectral replicas are determined by the spectral envelopes and each OAM mode's spectral envelope exhibits its characteristic fractional pseudo-Doppler shift.

These characteristic fractional shifts may be determined from information known at the receiver. For example, FIG. 6A illustrates a representative spectral envelope and replicas for OAM8, in which the fractional shift in the envelope is identified by the dashed vertical line and the spectral replicas are indexed by values of m.

FIG. 6B depicts a representative time-limited frequency-shift structure 600 for the recovery of OAM modes, in accordance with various aspects of the present disclosure. Structure 600 provides an exemplary model for recovering a desired OAM mode from the superposition of all OAM modes that appear at the gating output of the pseudo-Doppler modulation subsystem.

As depicted, the gating output is passed through a series of down-converters that perform frequency-shifts at multiples of 2F to baseband and the resulting M low-pass filtered baseband components are represented in vector X. It will be appreciated that, while frequency-shifts at multiples of 2F is disclosed, it is not intended to be limiting, as frequency-shifts at other multiples of F may be suitably employed.

The k-th row of coefficients {C_(k,m)} may then be used to recover the k-th order OAM mode from X, as the k-th entry of vector Y. Moreover, the entire broad-band spectrum of the gating output will contain spectral replicas of the transmitted signals located at the M harmonics of the pseudo-Doppler modulation frequency 2F. Further, it will be noted that the input and output signals of structure 600 are in the continuous-time domain and may also be in analog form.

In view of the above, the spectrum of each k-th order OAM mode at the gating output may be expressed as:

$\begin{matrix} {{Z_{G,k,1}(f)} = {{U_{k,{CS}}(F)}{\sum\limits_{m = {- \infty}}^{\infty}{S_{k}\left( {f - {m\; 2F}} \right)}}}} & (10) \end{matrix}$

where U represents the spectral envelope.

The values of U_(k) at spectral replica positions m can be arranged in a vector U of length M, and K such vectors determined for the K incident OAM modes. These may then be arranged column-wise in an M×K matrix U, and the input vector X_(M×1) to each of the K down-converting coefficient branches may be expressed jointly as:

X _(M×1) =U _(M×K) S _(K×K) A _(K×1)  (11)

where M≥K

Given these relationships, the incident OAM modes may be recovered by employing a pseudo-inverse of matrix U, as defined by:

U ^(#) _(K×M) =[U _(K×M) ^(H) U _(M×K)]⁻ U _(K×M) ^(H)  (12)

which is equal to matrix C, containing the baseband weighting coefficients {C_(k,m)} in FIG. 6B.

It then follows that a vector Y_(K×1) of output OAM modes of length K may be obtained by:

Y _(K×1) =U _(K×M) ^(#) X _(M×1) =S _(K×K) A _(K×1)  (13)

This is because each envelope vector comprising the columns of matrix U is generally linearly independent of K−1 of the other vectors and the greater M is (i.e. the more spectral replicas are included in matrix U), the higher is the likelihood of that being the case.

The entries of output vector Y_(K×1) may then be subsequently processed and equalized as in a conventional digital (i.e., QAM) receiver and demodulated into data streams. The demodulated data streams may then be recombined to form the final data output.

With this said, FIG. 8A depicts a high-level functional flow diagram of OAM signal recovery process 800, in accordance with various embodiments of the present disclosure. As shown, process 800 begins at task block 802, in which mode k OAM beam signals manifesting different data streams per each mode k are received by at least two antenna elements. The at least two antenna elements are separated by distance d along a circular locus having a radius R corresponding to the footprint area of the received OAM beams and operate to output antenna element signals in response to the received OAM beam signals.

At task block 804, the outputted antenna element signals are processed and combined by a variable ratio combining unit, in accordance with a high-rate periodic waveform having a frequency F that is greater than 2BR/d, where B is the common bandwidth occupied by the transmitted and received OAM beam signals. The high-rate periodic waveform operates to control the rapid switching between portions of the outputted antenna element signals to emulate unidirectional movement by a virtual, interpolated receiver antenna element along the circumference of the circular locus. The emulated receiver antenna element movement produces a fractional pseudo-Doppler frequency shift that results from its passage through the characteristic phase gradient of each OAM beam footprint along the circular locus.

The high-rate periodic waveform also serves to modulate and time-gate the outputted antenna signals so as to limit them to the time-intervals during which fractional pseudo-Doppler shift imparted to each of the received OAM modes is valid and unidirectional. In so doing, the variable ratio combining unit operates to proportionally combine the modulated, time-gated antenna element output signals to form a broadband output signal at its output ports Z₁, Z₂.

At task block 806, the combined modulated, time-gated antenna element output signals are shifted by multiples of frequency F and then low-pass filtered to generate baseband signals X_(m). At task block 808, the baseband signals X_(m) are each multiplied by a weighting coefficient C_(k,m) and then summed up to provide separate k-th OAM mode baseband signals Y_(k). And, at task 810, process 800 operates to apply same baseband signal vector X to all K rows of the complex-valued weighting coefficients in order to collect all OAM mode outputs in output vector Y.

At task block 812, process 800 operates to equalize and demodulate each of the K OAM mode signals Y_(k) and at task block 814, recombine the K demodulated data streams.

FIG. 8B depicts a comprehensive detailed functional flow diagram of the OAM signal recovery process 850, in accordance with another embodiment of the present disclosure. As shown, process 850 begins at task block 852, in which at least two suitably-disposed antennas simultaneously receives an RF signal composed of OAM beams modulated by separate data streams that are superposed in space and carrier frequency. At task block 854, the received antenna signals are coherently converted to baseband or a suitable intermediate frequency IF (a 0 Hz carrier is assumed). The converted baseband antenna signals are also simultaneously forwarded to task 857 for operations during training or calibration modes.

At task 856, the converted baseband antenna signals are modulated such that the amplitude of one signal is uniformly increasing while the amplitude of the other signal is commensurately decreasing during repeated time intervals at a rate F. At task 858, the modulated antenna signals are combined and may be gated in accordance with the above-designated time intervals synchronously with modulation rate F. Then at task 860, the combined timed-gated signals are shifted by multiples of F and then low-pass filtered to baseband.

As noted above, the converted baseband antenna signals are simultaneously forwarded to task 857 in which, during training/calibration operations, one of the converted baseband antenna signals is delayed by the delay of a variable-ratio power combiner (VRPC). At task 859, the VRPC-delayed baseband signal is subsequently correlated with the shifted baseband signals produced by task 860 and, in task 861, updated calibration coefficients C_(k,m) are obtained from the correlation results.

At task 862, the shifted baseband signals produced by task 860 are arranged in a row vector X, multiplied by the updated calibration coefficients C_(k,m) produced by task 861, and then summed to obtain a separated k-th OAM mode baseband signal. In task 864, the same baseband signal vector operation is applied to all K rows of weighting coefficients and the K OAM mode baseband signals are collected in output vector Y.

At task 866, each of the K OAM mode baseband signals are equalized and demodulated and, at task 868, the K OAM mode baseband signals are recombined

Extended OAM Pseudo-DOPPLER System Architecture

FIG. 7 depicts an extended VRPC unit 700 that also incorporates a gating unit at the output to emulate unidirectional antenna element motion, in accordance with various embodiments of the present disclosure. Extended VRPC unit 700 builds on the principles noted above to achieve greater sensitivity of the fractional pseudo-Doppler effect by incorporating multiple two-element pseudo-Doppler modulation subsystems to service more than two antenna elements.

In the illustrated embodiment, extended VRPC unit 700 is configured to service four antenna elements, RX1, RX2, RX3, RX4 by employing two first-stage two-element pseudo-Doppler modulation subsystems 702, 704, and a final-stage two-element pseudo-Doppler modulation subsystem 706. As shown, each of the modulation subsystems 702, 704, 706 embody the configuration of alternative VRPC unit 350 in which all of subsystems 702, 704, 706 are modulated synchronously with the same phase by the same source of Ω=2πF . The two first-stage subsystems 702, 704 each operate to process two of the outputted antenna element signals, respectively, and the outputs of the first-stage subsystems 702, 704 are subsequently fed to final-stage modulation subsystem 706.

By virtue of the system architecture of extended VRPC unit 700 that is configured to service additional antenna elements, the signal amplitudes of the additional elements may combine coherently while noise may combine incoherently to yield improved SNR. Moreover, incorporating a second output may be useful in providing at least one of some diversity in the OAM recovery process and the use of multipath signals.

It will be noted that, if the two first-stage modulation subsystems 702, 704 were again separated by “d”, the differences in their phases would appear in their ψ₁(t) phase terms and result in a double fractional pseudo-Doppler shift at the output of final-stage modulation subsystem 706 whose inputs they provide. This is because the separation of their respective antenna elements would to be 2d. Theoretically that would add to the first-stage systems 702, 704 shifts to triple the fractional pseudo-Doppler frequency shift at the final second-stage output. The process could then be iterated for more elements and more stages of the original subsystem.

However, by just moving the original two antenna elements of one subsystem from d to the same total span of 3d would effectively achieve the same result, so there would be no net gain in doing so. Therefore, sub-dividing the separations to d/3 could conceivably improve the SNR with the same net fractional pseudo-Doppler shift. That could allow operation closer to the OAM beam axis where SNR is lower, but because R would also be lower, the net fractional pseudo-Doppler shift would be increased.

Moreover, by coinciding the gating intervals for Z₁, Z₂, makes their fractional pseudo-Doppler shifts opposite in sign within the same span d, so the resulting difference would be twice the size of the shift at one output. That may be exploited to enhance the separability of closely-spaced OAM modes or in their recovered SNR, or in separating multipath components which will have negative corresponding OAM orders for odd number of reflections.

OAM Psuedo-DOPPLER Receiver Scheme and System Architecture Advantages

By virtue of the disclosed embodiments, the described receiver system architecture and scheme avoids the need to have large, complex receiving antenna structures designed to capture the entire circumferential phase progressions of the OAM beam signals. Moreover, the disclosed system architecture and scheme overcomes the susceptibility to low SNR as well as the limitation in range distances, and the need to precisely align the TX and RX antenna structures. It also affords a K×K MIMO functionality without requiring K antennas at the receiver, as only 2 antennas are required to recover any number K of OAM mode signals with this inventive scheme.

In view of these attributes and capabilities, the described receiver system architecture and scheme may be advantageously integrated into existing and future MIMO and massive-MIMO receiver infrastructures. 

1. An orbital angular momentum (OAM) receiver system, comprising: at least two receiver antenna elements configured to receive radiated OAM signal beams and generate antenna element output signals based on the received radiated OAM signal beams, the receiver antenna elements positioned tangentially along a circular locus and spatially separated by a distance d; the radiated OAM signal beams containing superposed order modes in which each of the order modes is denoted by integer k and a total number of superposed modes is denoted by integer K, wherein each of the K modes encompasses an individual stream of information data symbols; the circular locus having a radius R corresponding to a footprint area of the received radiated OAM signal beams, wherein the circular locus contains a characteristic phase gradient pertaining to each OAM beam along a circumference of the circular locus; and a variable ratio combining unit operative to receive and combine the antenna element output signals in time-varying proportions, the variable ratio combining unit configured to: switch between portions of the antenna element output signals in accordance with a high-rate periodic waveform of frequency F, the high-rate switch operation providing emulation of unidirectional movement by a virtual receiver antenna element along the circumference of the circular locus to produce a pseudo-Doppler frequency shift; modulate and time-gate the antenna element output signals in accordance with the high-rate periodic waveform to impart a fractional pseudo-Doppler shift to each OAM mode; and combine the modulated and time-gated antenna element output signals in accordance with the fractional pseudo-Doppler shift to facilitate separation of the OAM modes encompassing streams of information data symbols.
 2. The OAM receiver system of claim 1, wherein the frequency F of the high-rate periodic waveform satisfies a relationship: F>2BR/d, where B is a bandwidth of the received OAM signals.
 3. The OAM receiver system of claim 1, wherein the variable ratio combining unit comprises oppositely-adjusted variable phase shifting elements that are modulated by the high-rate periodic waveform.
 4. The OAM receiver system of claim 1, wherein the variable ratio combining unit comprises multiplying elements that are modulated by the high-rate periodic waveform.
 5. The OAM receiver system of claim 4, wherein the variable ratio combining unit comprises a synchronous time-gating unit that is controlled by the high-rate periodic waveform.
 6. The OAM receiver system of claim 1, wherein the variable ratio combining unit comprises at least one hybrid coupling element.
 7. The OAM receiver system of claim 1, wherein the modulated, time-gated antenna element output signals are shifted by multiples of frequency F then low-pass filtered to generate baseband signals.
 8. The OAM receiver system of claim 7, wherein the baseband signals are each multiplied by a weighting coefficient and then summed up to provide separate k-th OAM mode baseband signals.
 9. The OAM receiver system of claim 1, further comprising four antenna elements, two first-stage variable ratio combining units, and a final stage variable ratio combining unit wherein separation of the antenna elements corresponding to the two first stage variable ratio combining units is 2d.
 10. The OAM receiver system of claim 9, wherein the two first-stage variable ratio combining units and the final stage variable ratio combining unit are modulated synchronously with a same phase by the high-rate periodic waveform.
 11. A method for processing orbital angular momentum (OAM) signals, comprising: receiving, by at least two receiver antenna elements, radiated OAM signal beams containing superposed order modes in which each of the order modes is denoted by integer k and the total number of superposed modes is denoted by integer K, wherein each of the K modes encompasses an individual stream of information data symbols, the receiver antenna elements being positioned tangentially along a circular locus and spatially separated by a distance d, the circular locus having a radius R corresponding to a footprint area of the received radiated OAM signal beams wherein the circular locus contains progressive phase gradient information along a circumference of the circular locus; generating, by the receiver antenna elements, antenna element output signals based on the received radiated OAM signal beams; combining, by a variable ratio combining unit, the antenna element output signals in time-varying proportions; switching between portions of the antenna element output signals in accordance with a high-rate periodic waveform of frequency F, the high-rate switching operation providing emulation of unidirectional movement by the receiver antenna elements along the circumference of the circular locus to produce a pseudo-Doppler frequency shift; modulating and time-gating the antenna element output signals in accordance with the high-rate periodic waveform to impart a fractional pseudo-Doppler shift to each OAM mode; and combining the modulated and time-gated antenna element output signals in accordance with the fractional pseudo-Doppler shift to facilitate separation of the OAM modes encompassing streams of information data symbols.
 12. The method of claim 11, wherein the frequency F of the high-rate periodic waveform satisfies a relationship: F>2BR/d, where B is a bandwidth of the received OAM signals.
 13. The method of claim 11, wherein the variable ratio combining unit comprises oppositely-adjusted variable phase shifting elements that are modulated by the high-rate periodic waveform.
 14. The method of claim 11, wherein the variable ratio combining unit comprises multiplying elements that are modulated by the high-rate periodic waveform.
 15. The method of claim 14, wherein the variable ratio combining unit comprises a synchronous time-gating unit that is controlled by the high-rate periodic waveform.
 16. The method of claim 11, wherein the variable ratio combining unit comprises at least one hybrid coupling element.
 17. The method of claim 11, further comprising shifting the modulated, time-gated antenna element output signals by multiples of frequency F and low-pass filtering the antenna element output signals to generate baseband signals.
 18. The method of claim 17, further comprising multiplying the baseband signals by a weighting coefficient and summing up to provide separate k-th OAM mode baseband signals.
 19. The method of claim 11, further comprising providing four antenna elements, two first-stage variable ratio combining units, and a final stage variable ratio combining unit wherein the separation of the antenna elements corresponding to the two first stage variable ratio combining units is 2d.
 20. The method of claim 19, further comprising modulating the two first-stage variable ratio combining units and the final stage variable ratio combining unit synchronously with a same phase by the high-rate periodic waveform. 